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Abstract:

A slot resonance coupled, linear standing wave particle accelerator. The
accelerator includes a series of resonant accelerator cavities positioned
along a beam line, which are connected by resonant azimuthal slots formed
in interior walls separating adjacent cavities. At least some of the
slots are resonant at a frequency comparable to the resonant frequency of
the cavities. The resonant slots are offset from the axis of the
accelerator and have a major dimension extending in a direction
transverse to the radial direction with respect to the accelerator axis.
The off-axis resonant slots function to magnetically couple adjacent
cavities of the accelerator while also advancing the phase difference
between the standing wave in adjacent cavities by 180 degrees in addition
to the 180 degree phase difference resulting from coupling of the
standing wave in each cavity with the adjacent slot, such that the
signals in each cavity are in phase with one another and each cavity
functions as a live accelerating cavity. The resonance frequency of the
slot is the comparable to the resonance frequency of the cavities,
resulting in coupling of the cavities while also eliminating the need for
side-cavity or other off-axis coupling cavities.

Claims:

1. A slot resonance coupled standing wave particle accelerator
comprising:a hollow accelerator body having an elongate outer wall that
is substantially coaxial with a longitudinal axis that defines a beam
line, said accelerator body having a pair of transverse end walls at the
opposite ends of said outer wall and a plurality of spaced transverse
interior walls therebetween, said outer wall and said transverse end and
interior walls forming a plurality of accelerator cells positioned in
sequence along said axis, including a pair of reflective end cells
located at each end of said accelerator body adjacent said end walls,
each cell defining a resonant accelerator cavity in which a
radiofrequency standing wave may be maintained, and wherein said resonant
cavities have substantially the same resonant frequency;an input port
opening into at least one of said cells for introducing a high power
radiofrequency input signal operable to maintain a standing wave in said
accelerator body;adjacent pairs of said cells each sharing a common
interior wall, each interior wall having a resonant slot passing
therethrough that connects the resonant cavities on each side of said
common interior wall, each resonant slot being offset from said
longitudinal axis of said accelerator body and having a major axis
extending substantially transverse to the radial direction with respect
to said longitudinal axis of said accelerator body, each slot having a
resonant frequency comparable to said resonant frequency of said
cavities, said slots and said cavities having overlapping passbands such
that the frequency of said input signal may be selected to drive and
maintain the accelerator in a π/2 mode, such that adjacent cavities
are magnetically coupled by said resonant slots in said interior walls
and the passband associated with said standing wave is continuous in the
vicinity of said π/2 mode;each interior wall having a pair of nose
cones that extend from opposite sides of said interior wall into the
cavities on opposite sides of said interior wall, and each end wall
having a single nose cone extending into the cavity adjacent to said end
wall, each pair of nose cones extending into a cavity being opposed to
one another and terminating in tips which are spaced from one another to
form a gap between said tips, said nose cones and said transverse walls
having central bores that are aligned to form a beam tube that extends
the length of said accelerator body along said beam line and which has an
injection end and an emission end, through which charged particles may be
introduced, accelerated as they pass through said gaps, and emitted;said
cells being shaped and sized such that the distance between midpoints of
said gaps of adjacent cavities is approximately βλ, where
λ is the free space wavelength of the resonant standing wave in
said cavities and β is the velocity, normalized to the speed of
light, of a particle passing through said cavity; andwherein said end
cells are tuned such that said nodes of said standing wave occur in said
slots.

2. The slot resonance coupled standing wave particle accelerator defined
in claim 1 wherein said standing wave has a progressively increasing
phase velocity toward said emission end and is thereby maintained in
synchronism with charged particles as they are accelerated to higher
velocities along the length of the accelerator.

3. The slot resonance coupled standing wave particle accelerator defined
in claim 2 wherein said slots in adjacent interior walls are positioned
on opposite sides of said longitudinal axis from one another, such that
said slots are in alternating positions on opposite sides of said axis
along the length of the accelerator.

4. The slot resonance coupled standing wave particle accelerator defined
in claim 3 wherein each of said slots is semicircular in shape and
extends over an azimuthal range of between approximately 120.degree. and
180.degree. about said longitudinal axis.

5. The slot resonance coupled standing wave particle accelerator defined
in claim 2 wherein said gaps between said tips of said nose cones are
substantially centered longitudinally in said cavities.

6. The slot resonance coupled standing wave particle accelerator defined
in claim 5 wherein each of said nose cones has a length of at least
approximately 1/4.beta.λ as measured from the center of the wall
from which it extends.

8. The slot resonance coupled standing wave particle accelerator defined
in claim 2 wherein said transverse walls are spaced at increasing
distances from one another toward said emission end of said accelerator,
such that said standing wave has a progressively increasing phase
velocity toward said emission end of said accelerator and is thereby
maintained in synchronism with said charged particles.

9. The slot resonance coupled standing wave particle accelerator defined
in claim 8 wherein said cells of said accelerator body are of
progressively decreasing diameter toward said emission end so as to
maintain a substantially constant resonance frequency in said cavities
along the length of the accelerator.

10. The slot resonance coupled standing wave particle accelerator defined
in claim 1 wherein said input port opening into at least one of said
cells is located near the center of said accelerator body.

11. A slot resonance coupled standing wave particle accelerator
comprising:a hollow accelerator body having an elongate outer wall that
is substantially coaxial with a longitudinal axis that defines a beam
line, said accelerator body having a pair of transverse end walls at
opposite ends of said outer wall and a plurality of spaced transverse
interior walls therebetween, said outer wall and said interior walls and
said end walls forming a plurality of accelerator cells positioned in
sequence along said accelerator body, including a pair of reflective end
cells located at each end of said accelerator body adjacent said end
walls, each cell defining a resonant cavity in which a radiofrequency
standing wave may be maintained;adjacent pairs of cells each sharing a
common interior wall, each interior wall having a slot passing
therethrough that connects the resonant cavities on each side of said
interior wall, each of said slots being offset from said longitudinal
axis of said accelerator body and having a major axis extending
substantially transverse to the radial direction with respect to said
longitudinal axis of said accelerator body;each interior wall having a
pair of nose cones that extend from opposite sides of said interior wall
into the cavities on each side of said interior wall, and each end wall
having a single nose cone extending into the cavity adjacent to said end
wall, such that a pair of nose cones extends into each cavity, each pair
of nose cones extending into a cavity being opposed to one another and
terminating in tips that are spaced from one another to form a gap
between said tips, said nose cones and said transverse walls having
central bores that are aligned to form a beam tube that extends the
length of said accelerator body and which has an injection end and an
emission end, and through which charged particles may be introduced,
accelerated as they pass through said cavities, and emitted;selected ones
of said interior walls having a resonant slot having a resonant frequency
comparable to said resonant frequency of said cavities, such that
passbands associated with said cavities and said resonant slots overlap,
and selected other interior walls having at least one shorter nonresonant
slot that resonates at a frequency on the order of twice the resonant
frequency of said cavities, such that between each pair of interior walls
having said resonant slots there are n interior walls each having said
nonresonant slots, where n=1 to 4, and wherein the midpoints between
opposing nose cones in cavities connected by a resonant slot are spaced
by a distance of βλ and the midpoints between opposing nose
cones in cavities connected by nonresonant slots are spaced by a distance
of βλ/2;an input port opening into one of said cells for
introducing a high power radiofrequency input signal at a frequency
operable to maintain a standing wave in said accelerator body and to
drive and maintain said standing wave in a (n+1)π/(n+2) mode;wherein
the lengths of said nonresonant slots and the resonant frequencies of
cavities located between walls having nonresonant slots are selected to
obtain substantially equal accelerating electric field magnitudes in said
cavities, and the length of said resonant slots is selected so that the
dispersion curve for a periodic sequence of groups of (n+1) cavities is
continuous and has a non-zero slope in the vicinity of the
(n+1)π/(n+2) operating point; andwherein said end cells are tuned such
that said nodes of said standing wave occur in said slots.

12. The slot resonance coupled standing wave particle accelerator defined
in claim 11 wherein said standing wave has a progressively increasing
phase velocity toward said emission end of said accelerator and is
thereby maintained in synchronism with charged particles as they are
accelerated along the length of the accelerator.

13. The slot resonance coupled standing wave particle accelerator defined
in claim 12 wherein n=1 and wherein said slots in adjacent interior walls
are rotated by 90.degree. with respect to one another about said
longitudinal axis, such that successive resonant slots are in alternating
positions on opposite sides of said axis form one another along the
length of the accelerator, and such that said nonresonant slots are also
in alternating positions on opposite sides of said axis form one another
along the length of the accelerator.

14. The slot resonance coupled standing wave particle accelerator defined
in claim 13 wherein each of said resonant slots is semicircular in shape
and extends over an azimuthal range of between approximately 120.degree.
and 180.degree. about said longitudinal axis.

15. The slot resonance coupled standing wave particle accelerator defined
in claim 11 wherein said input port opening into one of said cells is
located near the center of said accelerator body.

16. The slot resonance coupled standing wave particle accelerator defined
in claim 12 wherein said transverse walls are spaced at increasing
distances from one another toward said emission end of said accelerator,
such that said standing wave has a progressively increasing phase
velocity toward said emission end of said accelerator and is thereby
maintained in synchronism with said charged particles.

17. The slot resonance coupled standing wave particle accelerator defined
in claim 16 wherein said accelerator body is tapered to a smaller
diameter toward said emission end to maintain an essentially constant
resonant frequency in said cavities.

Description:

BACKGROUND OF THE INVENTION

[0001]The invention disclosed and claimed herein is related to high energy
linear charged particle accelerators of the kind used to accelerate
protons, electrons or ions.

[0002]Linear particle accelerators are used to produce beams of
electrically charged nuclear or atomic particles. Low energy linear
particle accelerators include cathode ray tubes, x-ray generators, and
other similar devices. High energy linear accelerators, known as linacs,
are larger and more complex, typically ranging from approximately one
meter to several kilometers in length.

[0003]Linear accelerators are used in medicine for radiotherapy purposes
and in industry as testing electron accelerators and for other purposes.
They are also used in high energy nuclear physics research. Proton
accelerators, for example, are used as drivers for neutrino experiments
and as spallation neutron sources, and are of potential use in driving
and controlling sub-critical nuclear reactors. Another potential use of
high energy accelerators is the transmutation of radioactive nuclear
waste to benign nonradioactive elements.

[0004]A standing wave linear accelerator typically includes a series of
resonant cavities positioned along a longitudinal axis that defines a
beam line, which is the path of travel of the accelerated particles. The
cavities are connected by beam tube segments, which may be integral with
the cavities and which form a beam tube that opens into each cavity. The
cavities and the beam tube segments are electrically conductive,
generally being constructed of copper; and the entire beam line,
including the beam tube segments and the cavities, is evacuated.

[0005]The cavities are coupled to a power source that introduces a radio
frequency (RF) power signal into the cavities, typically a klystron that
produces a power signal in the microwave frequency range, to establish
and maintain a standing wave in the cavities. The standing microwave
signal provides the alternating electrical fields that accelerate charged
particles as they pass through each cavity.

[0006]As charged particles pass through the successive cavities along the
beam line, some or all of the cavities provide additional acceleration of
the particles. The particles are typically bunched so that they arrive at
the accelerating cavities in phase with the sinusoidally varying electric
fields in the cavities. The beam tube segments connecting the cavities
act as a Faraday cage, such that no acceleration occurs within the beam
tube segments. The combined acceleration of all of the cavities along the
beam line results in the particles being accelerated to their maximum
velocity and energy as they are emitted from the accelerator, which
velocity may approach but not exceed the velocity of light The ratio of
the velocity of an accelerated particle to the velocity of light is
generally represented as β, where β=v/c, and in many
applications a goal in designing an accelerator is to attain the highest
value of β as is feasible, given design and cost constraints.

[0007]Acceleration within a cavity is caused by the force of the
resonating electric field component of the standing wave acting on the
particles as they pass through the cavity. In order to achieve optimum
acceleration of a particular kind of particle passing along the beam
line, the sizes and shapes of the cavities, the spacing between cavities,
and the phases of the resonant signals within the cavities at each point
along the beam line, must all be selected so that the direction and
amplitude of the resonating electric field in each cavity are timed to
achieve maximum forward acceleration of the charged particles as they
pass through the cavity. In this regard, successive cavities along the
beam line are typically spaced apart by increasingly greater distances
toward the emission end of the accelerator, such that the distance
between any two adjacent cavities is the distance that a particle travels
during 1/2 period, or one period, depending on the phase shift of the
resonant standing wave from one cavity to the next at that point along
the beam line.

[0008]Early standing wave accelerators, i.e., those constructed before the
development of side-cavity coupled accelerators in the 1960's, were
either "0-mode" or "π mode" accelerators. The term "0-mode" has been
most commonly used to mean that there is a 0° phase shift in the
resonant RF signals from one cavity to the next. The term "π mode" has
been most commonly used to mean that there is a 180° phase shift
from cavity to cavity. These alternatives were used because other modes
have a shunt impedance that is smaller by a factor of two, and high shunt
impedance is a measure of the efficiency of an accelerator. This is
because the amplitudes of the fields in the cavities have a sinusoidal
distribution and all cells have the same phase, so only the cells at the
maximum of the sinusoidal distribution are optimally phased for
acceleration of the particles. The cavities near the nodes are
approximately 90° out of phase from the particles.

[0009]In the traditional π/2 mode, there is a 90° phase shift
from cavity to cavity, such that half of the cavities are unexcited and
thus do not effect any acceleration. Nevertheless, the problem with a 0
or π mode structure is that the dispersion curve at both 0 and π
has a slope of zero, so the mode separation is very small. With a small
mode separation a significant amount of the input power is dissipated by
exciting the modes adjacent to the desired mode, which do not contribute
to the acceleration of the particles. Furthermore, excitation of
undesired modes disturbs the desired electric field pattern and changes
the way the particles absorb energy, and thus disturb the synchronism
between the particle beam and the standing waves.

[0010]The π/2 mode is desirable because its dispersion curve, which
describes the phase advance per cavity as a function of the operating
frequency, is steepest and it has the largest inter-mode spacing for a
structure of a particular size. However, a standing wave accelerator must
be constructed with a larger number of cavities if adjacent cavities are
coupled with a π/2 phase advance, because in such an arrangement every
other cavity is a "dead," or nonaccelerating, cavity. A primary solution
to this problem has been to place the dead cavities off-axis, which
results in what is known as a side-cavity coupled accelerator. While the
dead off-axis cavities have little or no electromagnetic field, they
nevertheless couple the on-axis accelerating cavities together.
Side-cavity coupled accelerators, or any other cavity arrangement with a
π/2 phase advance, have the advantage of a reduced sensitivity to
construction tolerances. Also, such structures are phase-stabilized in
the sense that RF losses do not bring about phase shifts between the
accelerating cavities. Because of the symmetry associated with being in
the middle of the pass band, the π/2 mode has significantly more
relaxed tolerances than any other mode.

[0011]Most relatively recent proton accelerators have been constructed to
include three stages positioned in sequence along the length of the
accelerator: an initial radiofrequency quadrupole (RFQ) stage; a drift
tube linac (DTL) stage; and a side-cavity coupled linac (SCL) stage. The
transition from the DTL stage to the SCL stage is typically positioned at
a point in the accelerator at which the velocity of the accelerated
particles reaches a velocity of 0.4 to 0.5 times the speed of light, at
which the shunt impedance of the DTL and SCL linac stages are almost
equivalent.

[0012]The DTL technology was developed in the late 1940's and is still the
most widely used technology at lower beam velocities. Although DTL-based
proton acceleration linacs have been used for many years, they are
relatively bulky and difficult to service. They require a different RF
power source than the higher energy segments of the accelerator, and are
expensive to fabricate because of the need to incorporate quadrupole
magnet focusing cells within the drift tubes.

[0013]Accordingly, it is the object and purpose of the present invention
to provide a linear particle accelerator having accelerator cavities that
are simpler and less expensive to construct and service and, in
particular, which are free of side cavities or other off-axis coupling
cavities.

[0014]In particular, it is an object and purpose of the present invention
to provide a simpler linear particle accelerator structure that achieves
the foregoing objective and which is suitable for use in the high-energy
range that has previously been addressed with side-cavity coupled
structures, up to and including energy levels approaching the velocity of
light, or one MeV and above in the case of electrons.

[0015]It is also an object to provide a simpler linear particle
accelerator that may also be useful as well in the lower-energy range
previously addressed with DTL technologies.

SUMMARY OF THE INVENTION

[0016]The present invention provides a linear particle accelerator for
accelerating charged particles such as protons, electrons or ions, by
their interaction with a standing electromagnetic wave maintained within
the accelerator. Particles are introduced at one end of the accelerator
body, referred to as the injection end, and are emitted at the opposite
end, referred to as the emission end.

[0017]In one preferred embodiment the accelerator includes an elongate
hollow accelerator body having an outer wall that is generally coaxial
with the longitudinal axis of the accelerator. The accelerator body
further includes a series of transverse interior walls spaced
longitudinally along the beam line. Transverse end walls cap the
accelerator body at each end. The body and the walls are formed of copper
or other suitable conductor. A central longitudinal bore through the end
walls and the interior walls forms a beam line along which the charged
particles are accelerated.

[0018]The outer wall and the transverse walls form a series of accelerator
cells, each of which defines a resonant cavity in which a standing
electromagnetic wave may be maintained. The cells along the length of the
accelerator are sized and shaped such that their cavities have
substantially the same resonant frequency, while preferably also
providing an increasing phase velocity in the standing waves toward the
emission end of the accelerator, so that the standing waves in the
cavities are synchronized with the charged particles as they are driven
to increasingly greater velocities along the length of the accelerator.

[0019]Adjacent cells of the accelerator share a common interior wall. A
slot passes through each interior wall and connects the two resonant
cavities on either side. Each slot is offset from the longitudinal axis
of the accelerator and is oriented so that its major axis extends
transverse to the radial direction with respect to the longitudinal axis
of the accelerator. In one preferred embodiment each slot is semicircular
in shape, with its arc of curvature being centered on the longitudinal
axis of the accelerator, and with the slot extending through an azimuthal
angle of approximately 120 to 180 degrees. In this embodiment the length
of each slot is such that the slot itself has a resonant frequency
approximately equal to the resonant frequency of the cavities. Thus any
two adjacent cavities are coupled to the slot between them, and thus are
coupled to each other through the slot The slots, by resonating at a
frequency comparable to that of the cavities, function in a capacity
similar to that of the side cavities in a traditional side coupled
accelerator. This preferred embodiment is referred to as "biperiodic,"
meaning that a complete period consists of two resonators of comparable
resonant frequency, or one cavity and an adjacent slot.

[0020]The slots in adjacent interior walls are preferably positioned on
opposite sides of the beam line from one another, such that the slots
alternate in position along the length of the accelerator from one side
of the beam line to the other, in order to minimize direct coupling
between the slots in adjacent walls.

[0021]The accelerator body includes an input port in its outer wall,
preferably near its center, for introduction of a high power
radiofrequency input signal that produces and maintains the standing wave
along the length of the accelerator body. Since the accelerator is based
on use of a standing wave, no outlet port is necessary or desired.
Typically the accelerator is powered by a klystron, which introduces a
microwave signal having a power level of from hundreds of kilowatts to
tens of megawatts, through a waveguide connected to the input port of the
accelerator body.

[0022]By suitable selection of the frequency of the input signal and
tuning of the reflective end cells, the standing wave in the cells is
maintained in the π/2 mode, such that the resonant standing wave in
each cell is maintained in phase with the standing wave in the next cell,
and with their antinodes, or points of maximum electric field magnitude,
located in the cells and their nodes located in the slots. By so
maintaining the standing wave in the π/2 mode, a charged particle is
accelerated by the standing wave as it passes through each cavity. Thus
each cavity functions as a "live" accelerating cavity.

[0023]As noted, the charged particles increase in velocity as they are
accelerated, and synchronism between the particles and the standing wave
in each cavity must be maintained, for example by progressively
increasing the length of the cells along the length of the accelerator,
while also shaping the cells to maintain a constant resonant frequency.
Thus the phase velocity of the standing wave increases toward the
emission end of the accelerator tube while the standing wave also remains
in synchronism with the charged particles as they increase in velocity.

[0024]Each interior wall includes a pair of tubular nose cones that extend
from opposite sides of the wall into the two cavities on opposite sides
of the wall. Each end wall similarly includes a single nose cone
extending into the cavity adjacent the end wall. Each nose cone has a
central bore aligned with the bores passing through the transverse walls,
such that they define a beam tube that is interrupted by openings into
the central regions of the cavities. In the biperiodic embodiment the
tips of opposing nose cones extending into a cavity are spaced from one
another to form a gap, which is preferably centered longitudinally in the
cavity,,in order to concentrate and optimize the timing and effectiveness
of the alternating electric field in accelerating a particle as it passes
across the gap between the nose cones and through the center of the
cavity.

[0025]In a second preferred embodiment the accelerator is either
triperiodic or of a higher order of periodicity. In this embodiment some
of the interior walls have resonant slots, which resonate at a frequency
comparable to that of the cavities as described above, and other interior
walls have one or more shorter slots, which have a resonant frequency
distinctly higher than that of the cavities and the resonant slots. The
shorter slots are referred to herein as nonresonant slots to distinguish
them from the longer, resonant slots. The lengths of the nonresonant
slots are preferably selected so that they have resonant frequencies on
the order of twice the resonant frequencies of the cavities and the
resonant slots. As with the embodiment described above, the lengths of
the resonant slots are selected so that the passbands of the resonant
slots and the passbands of the cavities overlap.

[0026]In this second preferred embodiment, the interior walls having
resonant slots are separated by as many as four walls having nonresonant
slots. Further, the midpoints of the gaps between opposing nose cones in
adjacent cavities connected by a resonant slot are spaced by a distance
of βλ, while the midpoints of the gaps between opposing nose
cones in cavities connected by a nonresonant slot are spaced by a
distance of βλ/2. A suitable input signal is selected so as to
maintain a standing wave in the accelerator in the (n+1)π/(n+2) mode,
where n is the ratio of the number of interior walls having nonresonant
slots, to the number of interior walls having resonant slots.

[0027]Such an embodiment having two cavities for every resonant slot is
referred to as "triperiodic," which means that a complete period consists
of two cavities and an associated resonant slot, with the nonresonant
slots being disregarded for this purpose.

[0028]As with the first preferred embodiment, the end cells of the
accelerator are tuned to cause the nodes of the standing waves in the
cavities to occur in the slots. Also as with the first embodiment, the
phase velocity of the standing wave increases progressively toward the
emission end of the accelerator so as to be maintained in synchronism
with the charged particles as they are accelerated along the length of
the accelerator. This may be achieved by increasing the lengths of the
cells along the length of the accelerator tube toward the emission end.
The accelerator tube may be tapered in diameter to maintain a
substantially constant resonant frequency.

[0029]These and other aspects of the invention are more fully described in
the detailed description set forth below, taken with the accompanying
drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030]The accompanying Figures, when taken with the detailed description
of the invention set forth below, illustrate the construction and
operation of the present invention.

[0031]In the Figures:

[0032]FIG. 1 is an isometric view in partial cross section of one segment
of a preferred embodiment of a biperiodic linear particle accelerator
constructed in accordance with the present invention, with it being
understood that a number of segments similar to that shown in FIG. 1 are
positioned in sequence along the beam line of the accelerator;

[0033]FIG. 2 is a side view in cross section of the segment of the
accelerator shown in FIG. 1;

[0034]FIG. 3 is an end view in cross section of the segment shown in FIGS.
1 and 2, taken along section line 3-3 of FIG. 2;

[0035]FIG. 4 is a schematic partial illustration of a complete accelerator
containing segments such as those shown in FIGS. 1 through 3, such as may
be used for acceleration of electrons;

[0036]FIG. 5 is a schematic illustration of the peak electric fields in a
sequence of accelerator cells at various points in time, and showing the
synchronism between the peak electric fields and the position of a
charged particle as it travels through the cells;

[0037]FIG. 6 is a schematic side view two cavities connected by a slot,
and showing the electric and magnetic fields associated with a signal
resonating in the slot;

[0038]FIG. 7 is a schematic side view of the cavities and slot as shown in
FIG. 6, showing the magnetic and electric fields associated with resonant
signals in the cavities and the slot, in the π, π/2, and 0 modes;

[0039]FIG. 8 is a schematic partial illustration of a biperiodic
accelerator intended for acceleration of relatively heavy particles such
as protons or ions;

[0040]FIG. 9 is schematic side view in cross section of a triperiodic
accelerator constructed in accordance with the present invention, and
having interior walls with resonant as well as nonresonant slots; and

[0041]FIGS. 10 through 13 illustrate end views in cross sections taken
along corresponding section lines of FIG. 9.

DETAILED DESCRIPTION OF THE INVENTION

[0042]The present invention provides a linear standing wave particle
accelerator having a series of resonant accelerating cells connected by
slots, at least some of which slots are sized and shaped so as to be
resonant at a frequency comparable to that of the accelerating cells.

[0043]FIGS. 1 through 3 illustrate a segment 10 of a linear standing wave
accelerator constructed in accordance with a first preferred embodiment
of the present invention, which is referred to herein as "biperiodic" for
reasons explained below. The segment 10 includes a cylindrical copper
accelerator tube 12 having transverse interior walls 12a and 12b, which
together partially define three successive accelerating cells, or
cavities, 14, 16 and 18 (additional interior walls adjacent cavities 14
and 18 are not shown). Cells 14 and 16 share a common interior wall 12a,
and cells 16 and 18 share common interior wall 12b.

[0044]While the terms "cell" and "cavity" are generally used
interchangeably herein, the term "cell" refers more specifically to the
structures defined by the outer tube 12 and the various transverse walls
such as walls 12a and 12b, while the term "cavity" refers to the
volumetric spaces contained within those structures.

[0045]Each pair of cells along the accelerator tube 12 shares a common
transverse interior wall, as shown in FIGS. 1, 2 and 4, discussed further
below. It will be understood various cells along the accelerator tube 12
are functionally equivalent, and are structurally equivalent except with
regard to dimensional variations that are necessary to maintain a
constant resonant frequency for each cell along the length of the
accelerator tube 12, as also discussed further below. The number of cells
in the accelerator will depend on the purpose of the accelerator and the
type of particles accelerated (e.g., protons, electrons, or ionic
particles), but may be as many as several hundred cells.

[0046]Referring in particular to FIGS. 2 and 3, the three cavities 14, 16
and 18 are generally cylindrical in shape, with a central bore 20 passing
through the interior walls 12a and 12b. Central bore 20 defines the
nominal location of a beam line 22 that extends along the longitudinal
axis of the accelerator.

[0047]The internal transverse walls 12a and 12b include semicircular,
azimuthally extending resonant coupling slots 12c and 12d, respectively,
passing therethrough. Slot 12c connects cavity 14 and cavity 16, and slot
12d connects cavity 16 and cavity 18. Each slot 12c and 12d extends
through its associated interior wall over an azimuthal angle of between
120° and 180°, as viewed along the longitudinal axis of the
beam line 22 (FIG. 3).

[0048]The injection and emission ends of the accelerator tube 12 are
capped with transverse end walls 24 and 26 (FIG. 4), which are adjacent
reflective end cells 28 and 30, respectively. The beam tube bore 20
passes through end walls 24 and 26, but the end walls 24 and 26 do not
include coupling slots such as are formed in the interior walls. The
function of the reflective end cells 28 and 30 is to reflect a microwave
signal in the accelerator tube 12 and to thereby enable a standing wave
to be maintained in the tube 12, with the standing wave resulting from
the constructive interference of waves traveling in opposite directions
from one another. The end cells 28 and 30 are either half-cells or are
sized and otherwise tuned, by methods known in the art, to control the
phase of the reflected wave so that the nodes of the standing wave occurs
at the coupling slots in the interior walls.

[0049]Referring to FIGS. 1-3, the slots 12c and 12d in walls 12a and 12b
are essentially identical in size and shape, but are positioned on
opposite sides of the beam line 22. That is, they are rotated by 180
degrees relative to one another in the azimuthal direction, as viewed
longitudinally and as shown in FIG. 3, in order to compensate for dipole
kicks produced by the slots. Also, the 180 degree azimuthal offset of the
slots minimizes any direct coupling between slots in adjacent interior
walls. All of the slots connecting the various cells of the accelerator
structure are similarly positioned, so as to result in an alternating
azimuthal positioning of successive slots along the length of the
accelerator.

[0050]FIG. 4 illustrates in schematic form the accelerator as it may be
used for acceleration of electrons, for example in medical applications.
The accelerator includes the accelerator tube 12, an electron gun 32, and
a radio frequency power supply 34 which is connected to the accelerator
tube 12 by a waveguide 36. The electron gun 32 consists of any suitable
source of electrons available in the prior art, and preferably provides a
stream of electrons accelerated to an initial energy level of 10 to 50
KeV. The power supply 34 is a microwave power supply such as a klystron
or magnetron capable of producing an input signal having a power level of
at least hundreds of kilowatts. The accelerator tube 12 may include
typically from 10 to 100 cells along its length, which are connected by
coupling slots as described above.

[0051]The power supply 34 is preferably connected by waveguide 36 to the
accelerator tube 12 near the center of the tube 12 in order to optimize
the distribution of power in both directions along the length of the
accelerator tube 12. In operation, the power supply 34 provides the power
necessary to both maintain a standing wave along the entire length of the
accelerator tube 12 and to also accelerate electrons as they pass through
each cell in accelerator tube 12.

[0052]FIG. 4 also shows the reflective end cells 28 and 30 at opposite
ends of the accelerator tube 12 as being of different lengths, in
exaggerated proportion, to illustrate that the dimensions of the cells
may vary from one end of the accelerator to the other. Such a variation
is one way to synchronize the phase of the resonant standing wave in the
cells with the positions of the electrons passing through the cells as
they progressively increase in velocity along the length of the
accelerator. Thus the end cell 30 at the emission end of the accelerator
is elongated relative to the end cell 28 at the injection end, so that
the phase velocity of the standing wave increases toward the emission end
and the electrons passing through the cells thus remain synchronized with
the standing wave as they increase in velocity. A constant resonant
frequency may be maintained in cells of increasing length by, for
example, using cells of progressively smaller diameter toward the
emission end, as shown in the embodiment illustrated in FIG. 8 (discussed
below), which is not to scale.

[0053]FIGS. 5A through 5D illustrate the electric field component E of the
standing wave in a series of accelerator cells, as it varies over time as
a particle 38 passes through the cells. The electric field component E
extends longitudinally in both directions along the axis of the
accelerator and varies sinusoidally. The frequency of the input signal is
selected so that the standing wave resonates in what is referred to
herein as the π/2 mode. The resonating electric field components E in
the various cells are in phase with one another, and that the nodes of
the alternating electric fields are at the slots and the antinodes are
centered longitudinally in the cells. The frequency of the standing wave
is synchronized with the velocity of the particle 38 at the various
points along the accelerator, such that the electric field E in each cell
goes through one full cycle in the time that a particle 38 travels from
one cell to the next, for example from the position shown in FIG. 5B to
the position shown in FIG. 5D. The electric field E is at its maximum
strength in the forward direction as the particle 38 passes through the
center of a cell (FIGS. 5B and 5D), thereby accelerating the particle 38.
While the particle 38 travels through the nose cones between cells
(described below), the field E extends in the reverse direction but has
no effect on the particle 38 because the nose cones act as insulating
Faraday cages (FIGS. 5A and 5C). By the time the particle 38 arrives at
the next cell and is positioned in the gap between its nose cones, the
electric field E is again in the forward direction and thus further
accelerates the particle 38.

[0054]FIGS. 6A and 6B illustrate schematically a side view of the electric
field Es and the corresponding magnetic field Bs, of a signal
resonating within a resonant slot, taken 180 degrees apart in phase. Both
fields are illustrated as they would exist in the absence of any
resonating signal in the adjacent cavities. The electric field Es
varies sinusoidally and extends transversely to the longitudinal axis of
the slot, i.e., into and out of the plane of the paper as illustrated;
or, in the case of the accelerator, in the radial direction with respect
to the axis of the accelerator. The resonating magnetic field, Bs
also varies sinusoidally, wrapping around the axis of the electric field
Es and extending for some distance into the adjacent cavities on
opposite sides of the slot, where it extends in opposite directions with
respect to the major axis of the slot. Further, because the slot is
offset from the axis of the accelerator, i.e., to one side of the beam
line, the magnetic field Bs will interact with an azimuthal magnetic
field component of a resonant standing wave in either of the adjacent
cavities.

[0055]FIG. 7 illustrates such interactions schematically with regard to
three different modes. In FIG. 7, Bc represents the magnetic field
components of the signals in the two resonant cavities on opposite sides
of a resonant slot. FIG. 7A illustrates the 0 mode for a pair of cavities
connected by a resonant slot, FIG. 7B illustrates the π/2 mode for the
same cavities and slot, and FIG. 7c illustrates the n mode. In this
regard, the terms "0 mode," "π/2 mode" and "π mode" refer to the
mode in which a complete period extends from one resonant circuit to the
next, i.e., from one resonant cavity to the next resonant slot.

[0056]In both the 0 mode and the π mode, the azimuthally resonating
magnetic field components Bc of the standing wave in two adjacent
cavities are 180 degrees out of phase with one another and thus extend in
opposite directions at any point in time. Thus they may each extend
either in the same direction as the magnetic field component Bs that
extends into the same cavity in the 0 mode (FIG. 7A); or they may each
extend in the direction opposite to that of the component of the magnetic
field Bs extending into the same cavity in the π mode (FIG. 7c).
Thus, as long as the resonance frequency of the slot is comparable to the
resonance frequency of the adjacent cavities, a resonant signal in the
slot can coexist and couple with the resonant signals in the adjacent
cavities in either the 0 mode or π mode.

[0057]However, for the π/2 mode shown in FIG. 7B, the resonating
azimuthal magnetic fields Bc in the cavities extend at all times in
the same direction as one another and are in phase with one another, so
no significant resonant signal can coexist in the resonant slot, because
the magnetic field Bs of the slot would necessarily be in conflict
with one or the other of the magnetic fields Bc in the immediately
adjacent cavities.

[0058]Now turning to the function of the coupling slots in more detail,
each slot, taken alone, acts as a transmission line shorted at both ends
and thus has a resonance frequency associated with it. So long as the
length of the slot is equal to λ/2, where λ is the wavelength
of the resonant signal in the adjacent cavities, the slot itself is
capable of functioning as a resonator at the same frequency as that of
the cavities. Referring to FIGS. 1-3 and 5, since the major axis of each
slot extends in the azimuthal direction, the alternating electric field
Es in the slot can extend effectively only in the radial direction,
in order to satisfy the basic requirement of electromagnetic resonance
that an electric field must extend perpendicular to a reflecting
conductive surface. Thus, as described above, the alternating magnetic
field Bs, associated with a resonating signal in the slot, wraps
around the alternating electric field Es in the slot and extends
azimuthally alongside the slot in opposite directions on opposite sides
of the slot; while passing around the radial axis of the electric field
Es and through the slot at each end. Since the alternating magnetic
field Bs extends partially outside the slot and into the adjacent
cavities, it is capable of coupling with the alternating azimuthal
magnetic fields Bc in the adjacent cavities. Thus the resonant slots
are capable of magnetically coupling adjacent cavities.

[0059]Thus, for the standing wave components in adjacent cavities to
resonate in phase with one another, so that every cavity can function as
a "live," or accelerating cavity with every cycle of the standing wave,
the slots themselves cannot resonate with any significant signal
strength. Thus, so long as the standing waves in adjacent cavities are
balanced in strength and are in phase with one another, the resonant
signal in the slot between them is negligible and the slot acts as a
"dead" resonator, as shown in FIG. 7B, much the same as a side cavity in
a traditional side-cavity coupled accelerator. Thus in the π/2 mode
the slot effectively functions as a resonator only to couple adjacent
cavities and to correct imbalances between the resonant signals in the
adjacent cavities.

[0060]As a practical matter, there is in fact a small net traveling wave
component in the slots (not shown in the Figures), which transmits a
portion of the input signal power through the slots. Some transmission of
power through the slots is necessary to transmit sufficient power in both
directions along the accelerator tube, to compensate for power that is
dissipated at various points along the accelerator during operation
through ordinary losses as well as by acceleration of particles.

[0061]The operation of the slot resonance coupled accelerator as thus far
described can also be explained by comparison with the operation of a
conventional side-cavity coupled linac (SCL). In the SCL there is an
off-axis side cavity that couples each pair of neighboring on-axis
cavities. Each off-axis cavity, or coupling cavity, has two ports that
open into two neighboring on-axis cavities. The two coupling ports are
located on one side of the coupling cavity, such that a resonant magnetic
field in the coupling cavity couples to the on-axis cavities by means of
two field components that necessarily extend in the same direction. In
contrast, in the present invention the magnetic field components
associated with the resonant slot extend from opposite sides of the
resonant slot, and thus present to the two neighboring cavities as
magnetic fields extending in opposite directions. Thus, in the present
invention the electric fields in neighboring cavities are reversed in
direction relative to the situation in the SCL. This field reversal also
dictates the choice of gap-to-gap separation in order for a particle to
be synchronous with the standing wave. Whereas in the SCL the gap-to-gap
distance is βλ/2, in the present invention the gap-to-gap
distance is βλ.

[0062]Normally an accelerator tube constructed in accordance with the
present invention as thus far described has two passbands, one associated
with the resonant slots and one associated with the resonant cavities,
and with a stopband between them. Each passband represents a range of
frequencies which is readily transmitted through the cavities or the
slots. Achieving balanced and synchronized standing waves in the cavities
is accomplished by tuning the cavities and the slots so as to "close the
gap" between the passbands, or to superimpose the passband of the slots
on the passbands of the cavities, such that there is effectively only a
single passband associated with the standing wave, which passband is
continuous in the vicinity of the π/2 mode. This results in a standing
wave with the nodes in the slots having the same frequency as a standing
wave would have with the nodes in the cavities. Further, in the standing
wave accelerator of the present invention it is desirable to have a wide
passband, in order to maximize the frequency separation between the
π/2 operating mode and adjacent modes. Maximum separation between
these modes is desirable because it reduces leakage of power into the
adjacent modes, which do not significantly contribute to acceleration of
the particles and which distort the field profile, and thereby optimizes
the power efficiency of the accelerator.

[0063]The accelerator structure is designed to obtain a closed dispersion
curve, which is a curve that describes the phase advance per cell as a
function of the operating frequency. This is accomplished in the first
instance by using finite element analytical methods, using boundary
conditions selected to model a structure of semi-infinite length. To
obtain a closed dispersion curve, the operating frequencies of two π/2
modes are compared; the first with the cells active and the slots
inactive, and the second with the cells inactive and the slots active.
When both of these modes are tuned to the same desired operating
frequency and are thus equal to each other, the dispersion curve becomes
closed and the stopband no longer exists.

[0064]Further in this regard, it is notable that tuning to close the gap
between passbands is made easier by the fact that tuning the slot
frequency by varying the slot length has a strong effect on the π/2
mode when the slot is live, and very little effect on the π/2 mode
when the cavity is live. Conversely, tuning the cavity frequency by
changing the diameter of the cells or changing the gap length has a
strong effect on the π/2 mode with the cavity live, and little effect
on the π/2 mode when the slot is live. This is because changing any
dimension in a resonant structure changes the resonant frequency by an
amount proportional to the square of the field at the surface being
moved. Thus changing the length of the slot has almost no effect on the
frequency of the mode in which the slot is the node, and similarly
changing the diameter of the cavity has almost no effect on the frequency
of the mode in which the cavity is the node. The sign of the frequency
change for the electric fields is the opposite of the sign of the
frequency change for the magnetic fields, so it is preferable to tune the
surface where one or the other dominates.

[0065]As a result the microwave field strengths in the slots are
maintained at levels significantly less than the field strengths in the
cavities, and the resonating signals in the slots thus do not conflict
with the signals in the cavities.

[0066]Further, the reflective end cells are preferably tuned so as to
maintain "field flatness," i.e., to achieve equivalent on-axis peak
electric field strengths in all cells, or to otherwise tailor the
relative field strength along the length of the accelerator as may be
desired to achieve appropriate distribution of peak field strength along
the length of the accelerator.

[0067]Thus the slots function as resonators between the accelerating
cells, and in this regard the properly tuned slots are coupled to the
adjacent cavities and serve the same function as the side cavities of a
comparable side-cavity coupled linear accelerator. The present invention
makes side cavities unnecessary, yet with each cavity in the accelerator
tube functioning as a "live" or accelerating cavity.

[0068]Returning to FIGS. 1 through 3, the interior walls 12a and 12b each
include two integral, conical hollow nose cones, 12e and 12f, and 12g and
12h, respectively, which extend longitudinally in opposite directions
along the axis of the bore 20. The nose cones function to synchronize the
timing of the peak electric fields in each cell with the position of the
charged particles between the ends of opposing nose cones as they pass
through the cells, for example the region between the opposing ends of
nose cones 12f and 12g in cavity 16, and as shown in FIG. 5. The nose
cones also concentrate the electric field in the gaps between the nose
cones. This is desirable because the amplitude and direction of the
electric field in each cavity varies sinusoidally and thus acceleration
is optimized if the forward direction and the maximum field strength of
the electric field are timed to occur as particles are passing through
the center region of each cavity.

[0069]The preferred embodiment shown in FIGS. 1-3 is designed to operate
at a resonant frequency of approximately 805 Mhz. For such an operating
frequency the cavities have a nominal diameter of approximately 6.8
inches. For such cavities operating at a value of β of approximately
0.4, the cavity length is approximately 5.9 inches. The thickness of the
interior walls is approximately 1.0 inch and the bore 20 is approximately
1.5 inches in diameter. The nose cones are approximately 1.9 inches in
length and have large end diameters of 4.0 inches and a small end
diameters of 2.0 inches. For such a structure the optimum slot extends
over an azimuthal range of approximately 148 degrees and has inner and
outer radii of approximately 5.5 and 6.0 inches, respectively.

[0070]The structural elements shown in FIGS. 1-3 are illustrated as having
distinct edges and corners for purposes of illustration. However it will
be understood that in practice the internal edges and corners may be
rounded. Sharp edges in the vicinity of a large electric field result in
edge effects that concentrate the field and may cause electrical
breakdown. Rounding the edges raises the power level at which such
breakdown occurs and thereby allows the use of higher field strengths and
acceleration gradients. Similarly, rounding the internal edges and
corners in a region of high magnetic field minimizes the surface area
exposed to such field and thereby decreases RF losses. Rounding and
related modifications may be accompanied by adjustments in the geometry
of the cavity so as to maintain the desired resonant frequency.

[0071]As noted above, in the illustrated preferred embodiment the cavities
increase in length in the longitudinal direction along the length of the
accelerator, with each cavity having a length equal to βλ (see
FIG. 2 and 4), where λ is the free space wavelength of the standing
microwave signal used to accelerate the particles, and β is the
ratio of the velocity of a particle passing through the cell to the
velocity of light, as defined above. For low values of β of
approximately 0.2 to 0.5, this is an advantage, as it allows the cavities
to have a greater length, and therefore to have a higher shunt impedance
per cavity.

[0072]The embodiment of the accelerator thus far described is referred
biperiodic because a complete period consists of two resonators having
comparable resonant frequencies, i.e., one cavity and one slot, but has
neither side cavities nor any on-axis coupling cavities. Thus the
structure is almost axisymmetric, yet smaller than a comparable
side-cavity coupled accelerator because it has no side cavities.

[0073]As noted, the accelerator operates with a π/2 phase shift from a
cavity to the next adjacent slot, yet the cavities all have the same
phase and thus appear to be in the 0 mode. This is in fact analogous to
certain drift tube accelerators, which are stabilized by having λ/4
resonant stubs which couple from period to period. Such drift tube
accelerators also operate in the π/2 (90 degree) mode, but because the
magnetic field wraps around the resonant stubs there is a field reversal
and the accelerator thus operates in the zero mode.

[0074]As noted above, in the present invention the coupling slots are
sized and shaped so that the frequency of the π/2 mode when the nodes
are in the resonant slots is approximately the same as the frequency of
the π/2 mode would be if the nodes were in the accelerating cavities.
The fields thus have the same phase in every cavity, so the length of any
particular cavity must be βλ, where β=v/c is the particle
velocity normalized to the velocity of light, and λ is the free
space wavelength of the standing wave signal used to accelerate the
particles.

[0075]A common measure of the efficiency of an accelerator structure is
the shunt impedance per unit length, R, usually quoted in megohms per
meter. Shunt impedance is the ratio of the square of the energy gained
per meter, in MeV, to the power in megawatts dissipated per meter in the
structure. For cavities with nose cones, the shunt impedance is known to
increase with the cavity length up to a cavity length of λ/2.
Consequently, the accelerator of the present invention is more efficient
than side-cavity coupled accelerators for values of β less than 0.5,
and may be more efficient for values of β up to approximately 0.75.
This advantage may extend up to values of β as high as 0.75 because
the optimum cavity has nose cones that are λ/4 long, so the
periodic length is λ/2, in addition to the gap length (perhaps
βλ/4 or βλ/5) and the wall thickness (perhaps
λ/20). Yet the accelerator of the present invention is considerably
simpler and cheaper to fabricate.

[0076]As noted, the resonant slot coupling of the present invention
results in an additional 180 degree phase shift between the accelerator
cavities. This results in the magnetic fields in any two adjacent
accelerator cavities extending in opposite directions in both the 0-mode
and the π-mode, as shown in FIG. 7, which is opposite from the
situation in a comparable side-cavity coupled accelerator. This sign
reversal causes the two magnetic fields in the accelerator cavities to
point in the same direction at the π/2 phase advance point in the
dispersion curve. This is a critical difference between the function of
the slot-coupled accelerator of the present invention and a side-cavity
coupled accelerator. In addition, the fact that the accelerating cavities
all have the same phase has another benefit, which is that the currents
flowing on either side of a wall between cavities flow in opposite
directions, so the net current is zero. This means that the relatively
long slots required for resonance at the operating frequency actually
result in less loss than the much shorter slots in a side-cavity coupled
accelerator. The resonant slots of the present invention have the effect
of increasing the coupling by a factor of about 4 compared with typical
side-cavity coupled accelerators, which relaxes fabrication tolerances by
the same factor. Further, the frequency of the slot resonance is
primarily dependent only on its length and is relatively independent of
the slot width or the thickness of the wall. Thus the use of slot
resonance does not add wasted space to the structure. While the walls
between adjacent cavities need to be thicker for mechanical reasons to
accommodate the slots, simply because the slots weaken the walls
structurally, there are only half as many walls, so they can be twice as
thick and yet consume the same fraction of the accelerator length. Also,
the fact that there are half as many walls between cavities (because the
cavities are βλ long rather that βλ/2 long) means
that there is half as much resistive loss in the walls between cavities.
This, together with the fact that coupling losses are lower than for a
side coupled accelerator, may allow the shunt impedance of an accelerator
constructed in accordance with the present invention to be competitive
for values of β up to approximately 1.0.

[0077]As also previously noted, in order for the structure to be
synchronous with the particle beam, the longitudinal distance between
accelerator gaps is approximately βλ, as shown in FIG. 2. This
is twice the distance as that of a comparable side-coupled linac. For
β≈1, this is a potential disadvantage, as the available
spacing for each cell might be increased beyond the optimum. For low-beta
(0.2 to 0.5), however, this is an advantage, as it allows the cells to
occupy a greater longitudinal extent, and therefore have a better shunt
impedance per cell.

[0078]If the shunt impedance of the structure for β=1 is within 10 to
15% of the shunt impedance of a comparable side-cavity coupled
accelerator, a slot resonance coupled accelerator constructed in
accordance with the present invention may be up to 10 to 15% longer while
requiring the same amount of power, although it is nevertheless simpler
and thus less expensive to fabricate than a comparable accelerator having
side-coupled cavities or other on-axis coupling structures. By
eliminating side cavities, and because the cavities of the present
invention have a length of βλ, instead of βλ/2, the
number of machined parts is substantially reduced, nominally by a factor
of 4. However, for the same average gradient and gap length, the peak
fields will be twice as high as for a side-coupled linac.

[0079]FIG. 8 is a schematic illustration of an alternative preferred
embodiment of a biperiodic accelerator constructed in accordance with the
present invention, which is intended for acceleration of heavier charged
particles, such as ions or protons. The accelerator of FIG. 8 is intended
to represent a major accelerator and for such accelerators very high
power RF sources, on the order of tens of megawatts, are most economical
and thus a result in a more complex assembly. The accelerator assembly
includes an ion source 40 coupled to a radiofrequency quadrupole (RFQ)
42, which is in turn coupled to a drift tube linac (DTL) 44 powered by
one or more tetrodes 46. Tetrode RF sources tend to be more complex and
therefore more expensive because tetrodes tend to be lower gain and lower
power.

[0080]Charged particles emitted from the ion source 40 are initially
accelerated by the RFQ 42 and the drift tube linac 44 to a velocity on
the order of 0.2 c and are introduced into a series of accelerator tubes,
of which three tubes 48, 50 and 52 are shown. Accelerator tubes 48 and 50
may typically have as many as 100 accelerating cavities and the final
accelerator tube 52 may have as many as 50 cavities, all of which operate
essentially as described above with regard to the electron accelerator.
The number of cavities is mostly a matter of mechanical convenience since
the output end of each cell is on the order of one foot in length.
Representative reflective end cells from opposite ends of the series of
accelerator tubes are shown in exaggerated proportion as end cells 54 and
56. The three accelerator tubes are powered by three klystrons 58, 60 and
62. The klystrons as well as the tetrodes are amplifiers which are driven
by an RF driver 64, with a frequency multiplier 66 interposed between the
RF driver 64 and the klystrons 58 through 62. The enlarged cell 56 is
shown as being longer and thinner than cell 54, to illustrate
schematically one way to maintain substantially constant resonant
frequencies in the cells while also maintaining the standing wave in
synchronism with the accelerating particles. Particles emitted from the
accelerator may attain velocities on the order of 0.75 c.

[0081]As with the electron accelerator of FIG. 4 described above, the
cavities of the accelerator shown in FIG. 8 resonate at the same
frequency and are connected by resonant coupling slots 68, shown in the
enlarged cells 54 and 56 of FIG. 8. The cavities are shaped and sized to
maintain a constant resonant frequency, for example by making the
cavities near the emission end longer and narrower, while also
maintaining the standing wave in synchronism with the positions of the
charged particles as they are accelerated along the assembly.

[0082]Referring to FIGS. 9 through 13, there is illustrated a another
preferred embodiment of the invention, which is referred to here as a
triperiodic slot coupled accelerator, with the term "triperiodic" meaning
that a complete period consists of three resonators--two cavities and a
resonant slot. In this embodiment an accelerator tube 70 includes an
outer cylindrical wall 70a and two types of interior walls, 70b and 70c,
which in alternate in sequence along the axis of the accelerator. The
interior walls 70b and 70c are shown as being equally spaced from one
another so as to form cavities 72 of equal length, although it will be
understood that, as with the embodiment described above, the lengths of
the cavities 72 may increase progressively along the length of the
accelerator to accommodate increasing particle velocities.

[0083]The two types of interior walls 70b and 70c differ in the lengths of
the nose cones that extend in opposite directions from them. Interior
walls 70b have long nose cones 70d extending in each direction, whereas
interior walls 70c have short nose cones 70e extending in each direction.
For any particular particle speed, the spacing between adjacent walls 70b
and 70c is equal to the distance 3/4βλ. The lengths of nose
cones 70d and 70e are such that the midpoints of the gaps between
opposing nose cone tips in the various cavities are spaced apart by
alternating distances of 62 λ/2 and βλ, as shown in FIG.
9. The midpoints between opposing nose cones occur at the locations of
section lines 10-10, 11-11, 12-12, and 13-13; which section lines also
indicate the cross sections along which the cross-sectional views of
FIGS. 10 through 13 are taken.

[0084]As a result, it will be seen that the center-to-center distances for
the closely spaced (βλ/2) midpoints is one-half the
center-to-center distance between the widely spaced (βλ)
midpoints. For example, the distance between section lines 11-11 and
12-12 is one half the distance between section lines 10-10 and 11-11.

[0085]Referring to FIGS. 10 through 13, the alternating interior walls 70b
have long, resonant slots 74 and 76, whereas alternating walls 70c have
short, nonresonant slots 78 and 80. Further, the resonant slots 74 and 76
are offset azimuthally from one another on opposite sides of the
longitudinal axis of the accelerator 70; and the nonresonant slots 78 and
80 are likewise offset from one another azimuthally on opposite sides of
the longitudinal axis. Further, the resonant slots 74 and 76 are rotated
90 degrees about the longitudinal axis with respect to the nonresonant
slots 78 and 80. Thus the resonant slots alternate from top to bottom in
the Figures and the nonresonant slots alternate from left to right.

[0086]The embodiment of FIGS. 9 through 13 has a higher shunt impedance
for high values of β and is thus particularly useful for electron
accelerators at values of β greater than approximately 0.75. The
structure is axisymmetric except for the slots and all the interior walls
are of the same thickness.

[0087]In the triperiodic and higher-order embodiments, it is desirable to
maintain field flatness for the operating mode. In the triperiodic case
where n=1, one period consists of one "dead" resonant slot and two live
cavities, with the two cavities having the same RF field magnitude by
symmetry. In this case no special adjustment is necessary to assure that
all live cells have the same field strength. However, for n=2 or higher,
the cells connected by nonresonant slots will not necessarily oscillate
with the same amplitude. This can be corrected by adjusting the coupling
strength of the nonresonant slots, and possibly the resonant frequency of
the cavities. Achieving the desired coupling may require adding a second
nonresonant slot to the walls having nonresonant slots, positioned at a
180 degree azimuthal angle with respect to the first nonresonant slot.

[0088]Corresponding embodiments may be constructed with up to four walls
having nonresonant slots separating each pair of walls having resonant
slots. Depending on the number of walls having nonresonant slots, the
accelerator is operated in the (n+1)n/(n+2) mode, where n is the ratio of
the number of interior walls having nonresonant slots to the number of
interior walls having resonant slots.

[0089]While the present invention is described herein with reference to
certain preferred embodiments, it will be understood that various
modifications, substitutions and alterations may be made by one of
ordinary skill in the art without departing from the essential invention.
Accordingly, the scope of the present invention is defined by the
following claims.